It is known that foundry molds can be produced from sand, polyhydroxyl compounds and polyisocyanates (see, e.g. U.S. Pat. No. 3,499,861). It is also known that tertiary amines can be used as catalysts for accelerating the hardening of polyurethanes. In one known process for the production of foundry cores, a mixture of sand, polyhydroxyl compound and polyisocyanate is prepared in a storage vessel. The quantity of mixture required for producing the core is transferred to a core mold (core box). A mixture of air and a tertiary amine is subsequently blown through the core mold [R. Nagele: Erfahrungen mit dem Gas-Nebel-Verfahren (cold-box process) zur Kernherstellung, Giesserei, 56 (1969) No. 11, pages 298 et seq.]. Binders which have proved to be particularly suitable for this process include phenolformaldehyde resins of the novolak type and MDI-isocyanates. Triethyl amine has proved to be a suitable catalyst.
The disadvantage of this process lies in the limited storability of the mixture of sand, polyhydroxyl compound and polyisocyanate. The mixture has a maximum pot life of 1.5 to 2 hours, after which the mechanical strength of the cores produced therefrom decreases drastically so that they can no longer be used. If the mixture is stored in the storage vessel for a prolonged period (for example, overnight or over a non-working day), the mixture hardens. The storage vessel and all those parts of the core-shooting machine which come into contact with the mixture have to then be cleaned by a laborious operation.